1. Field of the Invention
The present invention relates to a discharge tube and, more particularly, to a discharge tube used as a light source such as a xenon short arc lamp, a mercury-xenon lamp, or the like.
2. Related Background Art
For example, the official gazette of Japanese Patent Application Laid-Open No. H01-213952 is a document that describes the technology concerning the discharge tube for effecting arc discharge between electrodes placed in a glass bulb. This gazette discloses the discharge tube in which an entire surface of a metal substrate is covered with a refractory metal like iridium so as not to expose a surface of a cusped tip of the metal substrate (emitter portion) containing an electron-emissive material like barium. The gazette also describes that it is feasible to stabilize the arc and decrease fluctuation of the arc, because the entire surface of the emitter portion is covered by a thin film of the refractory metal.
However, the technology described in the above gazette had the following problem. Namely, when the entire surface of the metal substrate containing barium is covered with iridium, barium cannot serve as an electron-emissive material at low operating temperatures. For this reason, the operating temperatures of the discharge tube must be kept high, so as to increase evaporation amounts of electrode materials, which will result in shortening the lifetime of the discharge tube.
The present invention has been accomplished under such circumstances and an object of the invention is to provide a discharge tube that can operate at low operating temperatures on a cathode for inducing arc discharge, thereby lengthening the lifetime.
In order to solve the above problem, the present invention provides a discharge tube in which a cathode with a cathode tip portion being fixed to a lead rod and an anode opposed to the cathode tip portion are encapsulated in a discharge gas atmosphere to effect arc discharge, wherein the cathode tip portion comprises a metal substrate of an impregnated type in which a porous, refractory metal is impregnated with an electron-emissive material or a sintered type in which a refractory metal containing an electron-emissive material is sintered, and a coating of a refractory metal which covers a predetermined portion in a surface of the metal substrate and which has a thickness of not less than 0.02 xcexcm nor more than 5xcexcm, wherein the metal substrate has a cusp pointed toward the anode, and wherein a tip portion of the cusp of the metal substrate is exposed without being covered by the coating.
In the discharge tube according to the present invention, the metal substrate of the cathode tip portion containing or impregnated with the electron-emissive material is covered in the predetermined portion by the coating of the refractory metal having the thickness of not less than 0.02 xcexcm nor more than 5 xcexcm, whereby the electron-emissive material is prevented from being evaporated in the coating part during operation of the discharge tube. On the other hand, the tip portion of the cusp of the metal substrate is exposed without being covered by the coating, which promotes emission of electrons from the electron-emissive material having diffused to the tip portion. For this reason, electrons can be efficiently emitted at relatively low temperatures, which can stabilize the discharge and which can also suppress the evaporation of the electron-emissive material, thus lengthening the lifetime. The inventors conducted intensive and extensive research and found that the lifetime of the discharge tube was able to be lengthened when the thickness of the coating covering the metal substrate was controlled in the range of not less than 0.02 xcexcm nor more than 5 xcexcm. Namely, when the thickness is smaller than 0.02 xcexcm, the coating reduces its effect of preventing the evaporation of the electron-emissive material. On the other hand, when the thickness is larger than 5 xcexcm, the coating becomes easier to peel off the metal substrate, so as to shorten the lifetime of the discharge tube.
The thickness of the coating is desirably selected in the range of not less than 0.2 xcexcm nor more than 3 xcexcm. In this case, it becomes feasible to further enhance the effect of preventing the evaporation of the electron-emissive material and almost nullify the possibility of peeling-off of the coating from the metal substrate.
The present invention will become fully understood from the detailed description and accompanying drawings which will follow. It is to be considered that these are presented merely for illustration of the invention but do not limit the present invention.